SAN FRANCISCO -- Hoping to reduce the risk of major oil spills in San Francisco Bay, the Coast Guard and top shipping officials Thursday passed new rules to restrict cargo ships, oil tankers and other large vessels from sailing near the Bay Bridge in heavy fog.

The action comes five weeks after an empty oil tanker, the Overseas Reymar, sideswiped a tower of the Bay Bridge near Yerba Buena Island. The Jan. 7 accident, which caused an estimated $3 million in damage to the bridge, occurred five years after the Cosco Busan, a 901-foot cargo ship, hit an adjacent tower. That accident spilled 53,000 gallons of bunker fuel into the bay, killing more than 6,000 birds and oiling 69 miles of beaches and shoreline.

Under the new guidelines, all large ships will be restricted from sailing northbound under the Bay Bridge when visibility is less than half a mile.

Both the Overseas Reymar and the Cosco Busan hit the bridge while sailing north in fog conditions where visibility was roughly a quarter mile.

The rules, which will be enforced by the Coast Guard and take effect immediately, apply to ships sailing out of the bay. They do not apply to ships sailing from the ocean into the bay because of concerns that broader regulations could stall commerce.

"We were responding to the casualty of the Overseas Reymar," said Cynthia Stowe, the Coast Guard's captain of the Port of San Francisco. "We're taking a very conservative approach."

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Deb Self, executive director of the environmental group Baykeeper, praised the decision.

"I'm very pleased,'' she said. "It should help reduce the risk of oil spills."

Every year, roughly 3,500 cargo ships, oil tankers, chemical ships and other large vessels sail in and out of San Francisco Bay.

Biologists have said that if a large oil tanker spills its cargo in the bay, the currents could carry much of the oil southward and to the East Bay shoreline, where it would devastate egrets, herons, harbor seals, salmon and other species in the marshes and wetlands. Because of the weak tidal action in the southern part of the bay, the oil would take months, if not years, to remove. Costs for cleanup, lost tourism and other damage could easily eclipse $1 billion.

The new rules were approved unanimously Thursday morning by the San Francisco Bay Harbor Safety Committee at its monthly meeting on Pier 1 along the San Francisco waterfront. The 23-member committee, established by state law following the 1989 Exxon Valdez spill, is made up of shipping industry officials, Coast Guard leaders, pilots and environmentalists.

A tanker ship brushed up against one of the Bay Bridge towers in January. (KGO photo)

The rules Thursday were described as "temporary" but are expected to be made final after the Coast Guard finishes its investigation into the Overseas Reymar accident. The final report is several months away, Stowe said.

After the 2007 Cosco Busan spill, which federal investigators said was caused in part by pilot John Cota sailing in dense fog, the Harbor Safety Committee came up with a list of nine areas where large ships should not sail when visibility is less than half a mile. These "critical maneuvering areas" included the San Mateo Bridge, Richmond-San Rafael Bridge and Oakland Inner Harbor. But, as the Bay Area News Group reported in January, the Bay Bridge was deliberately left off that list.

Capt. Peter McIsaac, president of the San Francisco Bar Pilots, said in an interview last month that he and Coast Guard officials helped craft the fog rules in 2008. The Bay Bridge was not included among the areas to be avoided in fog, he said, because foggy conditions are so common in San Francisco Bay that limiting all sailing near the Bay Bridge would bring commerce to a near halt.

Oil tankers, large cargo ships and other vessels regularly park at an area called "Anchorage 9" just south of Yerba Buena Island -- visible to baseball fans from the stands at the Giants' AT&T Park -- where the vessels are refueled, inspected and loaded with food for crews.

If inbound ships in fog can't sail under the Bay Bridge to get to Anchorage 9, "you could end up with six or eight ships out there turning doughnuts off the Marin coast, and that wouldn't be safe," McIsaac said last month.

Environmentalists and a prominent shipping industry group, the Pacific Merchant Shipping Association, however, said the rules needed to be rewritten in light of two large ships hitting the Bay Bridge in five years.

The rules passed Thursday also require the captains of large ships and the local pilots that help guide them to:

Sail under the wider spans of the Bay Bridge -- between the A and B towers or the D and E towers -- when visibility is limited because of fog.

Immediately report visibility changes to the Coast Guard when ships are moving.